Recently, it has been proposed to mount electronic components more densely in each circuit board. In particular, the recent tendency has been toward decreasing the number of solder bumps per circuit board and reducing the package size.
Along with the considerable decrease in size of the solder bumps or balls, the demand for a higher reliability on the mechanical strength of the solder joint has also become more stringent. In the recent field of this industry, as the computers have been improved very much in function, simulation of the possible stress in the printed circuit boards used in the computers has been pursued more actively. Although the manufacturers of electronic devices have actively been trying to measure possible strain in such electronic devices, it is still difficult to attain a coincidence between the simulated strain value and measured strain.
For example, the Japanese Patent Laid-Open No. 2001-15882 discloses a circuit board having a strain gauge built therein. As disclosed in this document, information on an actual stress which will possibly lead to rupture or destruction of the circuit board can be acquired by measuring the strain of the solder joints at the circuit board surface and influence of strain rate when the circuit board has been bent or applied with an impact or shock. Currently, however, it is difficult to measure the amount of strain of the solder joint. On this account, a strain gauge is disposed as near the solder joint as possible to measure the strain and the measured solder joint strain and rupture are correlated with each other.
However, since actual stress given to a circuit board cannot accurately be measured only with the information on strain of the surface layer, it is necessary to measure strain inside the circuit board. On this account, the Japanese Patent Laid-Open No. 2000-340916 proposed a circuit board in which a strain measuring metal piece highly variable in resistance is buried and electrically connected through a through-hole.
However, the above-mentioned conventional circuit boards are disadvantageous in that the resistive element is broken due to a crack developed in the insulative resin layer when strain developed due to a given impact during the impact resistance test is measured and that no accurate measurement is possible since strain occurs due to a through-hole formed to electrically connect a metal foil provided in the circuit board and measuring electrodes provided in the surface layer of the circuit board to each other.
The present invention has an object to overcome the above-mentioned drawbacks of the related art by providing a multilayer printed circuit board and a testing piece for the printed circuit board, having such an impact resistance that no crack will take place in an insulative resin layer and a resistive element will not be broken and capable of accurate strain measurement.